The present invention concerns a filtering unit as defined in the introductory part of claim 1. An important area of application of filtering units are the fat filters used in kitchens. A fat filter separates fat and other impuritites from the vapours and fumes produced in connection with cooking, thereby preventing these detrimental and inflammable substances from getting into the ventilation system. The fat filter is usually placed in an air exhauster mounted above the cooker or equivalent. In recent times, it has also become common to mount a fat filter in an air exhaust duct in the ceiling of the kitchen. The purpose of fat filters is to keep the air ducts and fans clean and to improve fire safety and hygiene. The requirements on a good fat filter are a high filtering efficiency, easy maintenance, even pres¬ sure drop during the whole cleaning interval, fire safety, good hygienic properties and preservation of a high filtering efficiency even after numerous washing operations.

Among the fat filters currently used are the so-called collision filter and the so-called centri- fugal filter. A collision filter consists of metal wires or a metal meshwork. The use of these filters has been clearly declining because they do not meet the require¬ ments imposed on fat filters. Their worst drawbacks are the danger of fire, change of pressure drop during use and a short service life.

The worst problem with centrifugal filters is their relatively low filtering percentage. The best centrifugal filters can separate 80 % of 5-μ particles. However, to prevent the air ducts from getting greasy, the filtering percentage must be at least 80 % for 3-μ particles and over 98 % for 5-μ particles. The low filtering percentage of currently used centrifugal

filters is due to imperfections in their construction. Examples of this kind of coarse filters are found in patent publications FI 77164, DE 2718611, DE 2731996, IT 471267, US 3813856, US 3834135 and US 3910782. In practice, the filtering percentage of such filters varies greatly depending on the magnitude of the air current. Also, deficient technical design regarding the flow dynamics results in noise problems.

The cyclone is a coarse filter commonly used in industry. Depending on the size and ratings of the cyclone, it can reach a filtering percentage of up to 90 % for 2-μ particles. Examples of cyclone applications are US-patents 3948771 and 4375365. Fl-patent 77163 presents a filter which is supposed to achieve a better filtering efficiency without using rotary motion, but the results have not improved significantly. This is due to the disadvantageous construciton of the separat¬ or.

There are also so-called two-stage filters, consisting of two successive filters. Such filters have a good filtering efficiency, but, due to problems re¬ garding maintenance and fire safety as well as a high price, they have not been widely accepted.

The object of the present invention is to eliminate the drawbacks referred to above. A specific object of the invention is to achieve a filter that is better and cheaper than previously known filters employ¬ ing the centrifugal prinicple. More specifically, the invention aims at a filtering unit that is simple and cheap to manufacture and whose filtering percentage is such that the air ducts and fans will not get dirty. The invention also aims at a filtering unit in which the pressure drop will not change substantially during use, and which is easy to maintain, presents no fire hazard and is usable as a ceiling exhauster, prolonging the service life of the fine filters possibly used in conjunction therewith.

As for the features characteristic of the invention, reference is made to the claims.

The filtering unit of the invention comprises at least one cylindrical and essentially vertical separ- ating chamber with inflow and outflow openings. Accord¬ ing to the invention, the inflow opening is a longitud¬ inal, i.e. vertical aperture in the cylindrical wall of the separating chamber, said aperture opening essential¬ ly tangentially into the chamber and increasing in width in the downward direction. The inflow opening is prefer¬ ably placed closer to the top end of the chamber than to its bottom end.

The wall of the separating chamber is prefer¬ ably provided with several similar inflow openings placed at about the same height and at even distances around the chamber wall.

In a preferable embodiment of the invention, the filtering unit comprises several adjacent vertical separating chambers connected together to form a suit- able separating chamber module constituting a solid wall, the individual chambers being joined together e.g. with a suitable plate connecting the chambers by their tops, said plate enabling the unit to be mounted on the inlet opening of an air exhaust system. In addi- tion, the separating chambers are connected together by. their bottoms by means of a suitable basin receiving the fat and other impurities separated from the air flow and running down the chamber walls.

The basic principle of the filtering unit of the invention is that the cylindrical separating chamber is provided with at least one, but preferably several inflow openings whose width increases towards the lower end, which means that most of the air flowing into the separating chamber enters the chamber through the lower part of the inflow opening. Since the inflow openings are so designed that they direct the air flow tangen¬ tially into the chamber, the air in the chamber will be

set into a rotating motion and most of it will be di¬ rected at first towards the bottom of the chamber, rotating along the interior surface of the chamber wall. In the central region of the lower part of the separat- ing chamber, the air flows upwards towards the outflow opening. The air entering the chamber through the upper part of the inflow opening also gets into a rotating motion but is directed upwards towards the outflow opening. The fact that the inflow opening tapers in the upward direction means that essentially the main filter¬ ing mechanism in the region of the upper end of the opening is based on the impact effect, whereas in the region of the lower end of the inflow opening the fil¬ tering mechanism is based on the rotation of the air current. The fact that the inflow openings have, a taper¬ ing form and are placed all around the separating cham¬ ber means that the boundary layers are thin, resulting in a better separating efficiency and an essential reduction in the amount of air flowing directly into the outflow opening.

Advantageous dimensions of the separating chamber as determined on the basis of experiments are: diameter below 100 mm, preferably 45 - 70 mm, height below 350 mm, preferably 180 - 230 mm, and width of inflow opening at the lower end 10 - 14 mm and 2 - 4 mm at the upper end.

Among the advantages of the filtering unit of the invention as compared to previously known techniques are a simple construction and consequently low manufac- turing costs, easy cleanability, good separating charac¬ teristics, noiseless operation and free constructional adaptability enabling the unit to be fitted to various physical environments.

In the following, the invention is described in detail by referring to the appended drawings, where¬ in: fig. 1 presents an embodiment of the filtering unit of

the invention as seen from below, fig. 2 presents the unit in fig. 1 as seen from above, fig. 3 illustrates the inflow openings of the separating chamber, fig. 4 shows a cross section of the separating chamber, fig. 5 shows how the separating chamber is connected to a fine filter, fig. 6 presents a separating chamber module as provided by the invention, and fig 7. presents a fine filter as provided by the inven¬ tion.

An embodiment of the filtering unit of the invention as illustrated by figs. 1 and 2 comprises four adjacent separating chamber modules 5, each module being connected to an air exhaust channel inlet 12 via a fine filter 8 placed between the channel and the module.

As shown in fig. 6, the separating chamber module consists of five adjacent cylindrical separating chambers 1 joined together by their tops by means of a top plate 6 provided with an outflow opening 3 for each chamber, each opening 3 being essentially concentric with the chamber. In addition, the module is provided with a bottom basin 10 which receives the impurities separated in the chamber and running down from it. The wall of the upper part of each separating chamber is provided with inflow openings 2, whose structure and action will be described in detail in connection with figs. 3 and 4. Fig. 7 shows a fine filter 8 which may be used in the filtering unit of the invention. It consists of a box having the form of an essentially rectangular parallelepiped and filled with suitable fine filtering material 13 known in itself. The upper and lower sur- faces of this filtet box are provided with large open¬ ings for the flow of the air to be filtered. As illus¬ trated by fig. 5, the cover 14 of the fine filter box

protrudes over the longer side walls, forming shoulders 15 enabling the fine filter to be secured on mounting rails 7 provided in the air exhaust channel structure 12. The edges of the bottom of the fine filter box are bent down so as to form mounting rails 9 which corre¬ spond to rails 7 and permit the separating chamber module to be similarly attached by its top plate 6 to the fine filter. Since the cover 14 of the fine filter and the top plate 6 of the separating chamber module 5 have essentially the same dimensions while mountig rails

7 and mounting rails 9 have corresponding dimensions, the filtering unit of the invention may consist of only separating chamber modules 5 connected directly to the air exhaust channel 12, or, if fine filtering is desir- ed, it may additionally be provided with a fine filter

8 mounted between module and channel as illustrated by fig. 5.

Figs. 3 and 4 illustrate the construction and operation of the separating chamber 1 of the invention. The separating chamber, consisting of a cylindrical structure of a round cross section, is provided with inflow openings 2 located in its upper part. In this embodiment, these openings have been formed by bending parts 11 of the chamber wall inwards as follows. A vertical incision 16, i.e. an incision parallel to the axis of the separating chamber, has been made in the chamber wall, and a shorter transverse incision 17 has been made at the . lower end of the first incision. The tongue-like part 11 of the chamber wall has then been bent somewhat into the separating chamber along the line 18 connecting the non-coincident ends of the two incisions. In this way, the chamber wall is provided with a slit whose width increases steadily in the down¬ ward direction and which opens downwards in the region of the transverse incision 17. Thus, the slit is direct¬ ed essentially tangentially to the interior surface of the chamber.

The inflow opening 2 of the separating chamber of the invention acts as follows. As the suction in the air exhaust channel creates a vacuum effect in the separating chamber 1, an air flow as indicated by the arrows 19 is set up at the inflow opening 2. In the narrowest region of the opening 2, i.e. in its upper¬ most part, the air flow is of a relatively small volume and is directed essentialy horizontally. The volume of the air flow increases steadily in the downward direc- tion along the slit 2, and simultaneously its direction turns steadily downwards. Thus, the largest and strong¬ est air flow occurs in the lower part of the opening 2 and is directed towards the lower part of the separat¬ ing chamber, where it forms a centrifugal flow rotating along the chamber wall. In this manner, the separating chamber acts mostly as a centrifugal separator while the air rises upwards through the central part of the separating chamber. In the upper part of the separating chamber, the main filtering mechanism is based on the impact effect.

As illustrated by fig. 4, the separating cham¬ ber module 5 is preferably constructed from two plates 20 and 21 which are provided with the shapes correspond¬ ing to the two halves of the separating chamber with inflow openings and attached together against each other. Thus, this construction allows separating chamber modules of different lengths and shapes to be produced according to need and in a manner as cheap and simple as possible. Although the invention has been des¬ cribed in the foregoing by referring to the appended drawings as an example, various embodiments are possible within the scope of the idea of the invention as defined in the claims.